4 Simultaneous Channels Okay For 802.11b

Note: Last summer, before starting here at Ziff Davis Media, I did some work with a small 802.11b startup called Cirond. The following story is based on a white-paper analysis done by CTO Mitch Burton. After I read it, I wanted to tell the world. I’ll make no claims on whether Cirond’s forthcoming products are useful, but its Channel Overlap analysis is exciting. But in the interest of full disclosure, note that they did pay me for some of the work I did  although that was almost six months ago. So take this story with the proverbial grain of salt. I find it exciting, you may not.

Current thinking in the 802.11b wireless space is that only three of the 11 channels used by wireless hubs in the US can be allocated simultaneously. But that’s wrong, according to Mitch Burton, CTO of Cirond Networks. In fact, 4 of the 11 channels in North America, and 5 of the 13 in Europe can be safely used without significant interference or crosstalk  and this has significant ramifications for multi-access point deployments.

In North America, the 802.11b spectrum ranges from 2400MHz to 2483MHz, and is divided up into 11 channels from 2412MHz to 2462MHz, spaced 5MHz apart. Thus However, each channel is 22MHz wide, so as you can imagine, there is great overlap. Channel 1, for instance, is centered at 2412MHz, but extends out from 2401MHz to 2433MHz. Channel 6 is centered at 2437MHz, extending from 2426MHz to 2448MHz.

In a multi-access point installation, where overlapping channels can cause interference, dead-spots and other problems, Channels 1, 6 and 11 are generally regarded as the only safe channels to use. Since there are 5 5MHz channels between 1 and 6, and between 6 and 11, or 25MHz of total bandwidth, that leaves three MHz of buffer zone between channels.

Note that wireless access points generally radiate waves in a sphere around the access point, attenuated by walls, cubicle material, ceilings and floors. With just three channels to work with, it can become difficult to deploy wireless access around a single or multi-floor location while only reusing those 3 frequencies

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